The present invention relates to a light-storing fluorescent pigment and, more particularly, to a light-storing fluorescent pigment containing a mother crystal doped with a rare earth metal.
The light-storing fluorescent pigment of the present invention can be advantageously used as a display means of various apparatuses, measuring instruments and members because of its excellent light emitting characteristics, easy production, lack of contamination with impurities and ease of control of the particle diameter of pigment particles. The present invention also relates to a method of producing such a light-storing fluorescent pigment. As recognized in this technical field, the xe2x80x9clight-storing fluorescent pigmentxe2x80x9d refers to a fluorescent pigment capable of maintaining afterglow enough to be visually recognized for a time of from tens of minutes to several hours even after the termination of stimulation when light is emitted by stimulating the pigment using ultraviolet radiation.
Various light-storing fluorescent pigments have been suggested and can be actually purchased and used. Among them, a popularly used light-storing fluorescent pigment is a zinc sulfide-based light-storing fluorescent pigment, that is, those obtained by adding a radioactive substance to a zinc sulfide-based fluorescent substance (ZnS:Cu) to enhance the light-storing performance. This fluorescent pigment can emit light for a long time using the energy of the added radioactive substance. However, this fluorescent pigment has such a drawback that it is chemically unstable and is inferior in light resistance. Accordingly, its application is limited to a narrow range. Furthermore, this fluorescent pigment has such a drawback that production control is difficult and the amount is limited because a radioactive substance is contained.
To solve the problems of a conventional zinc sulfide-based light-storing fluorescent pigment, Japanese Unexamined Patent Publication (Kokai) No. 7-11250 suggests a light-storing fluorescent substance, which is characterized by using a compound represented by MAl2O4 (M is at least one metal element selected from the group consisting of calcium, strontium and barium) as a mother crystal, and adding europium as an activator and an element such as lanthanum as a coactivator. According to this invention, there can be obtained a light-storing fluorescent substance which has afterglow characteristics with high luminance for a time longer by far than that in the case of a commercially available sulfide-based fluorescent substance, and which is chemically stable and is also superior in light resistance. Since this light-storing fluorescent substance does not use a radioactive substance, the drawbacks caused by its use can be avoided.
U.S. Pat. No. 5,376,303 discloses a light-storing fluorescent substance comprising a composition represented by the following formula:
MOa(Al1xe2x88x92bBb)2O3:cR
(wherein MO represents at least one divalent metal oxide selected from the group consisting of MgO, CaO, SrO and ZnO; and R represents Eu and at least one additional rare earth element selected from the group consisting of Pr, Nd and Tm, and the rare earth element is contained in the amount enough to enhance afterglow characteristics). Also according to this invention, the same effect as that of Japanese Unexamined Patent Publication (Kokai) No. 7-11250 described previously can be obtained.
By the way, problems to be solved are still present in these light-storing fluorescent substances free from radioactive substance. A first problem is a problem caused by using a solid state reaction method in the production of the light-storing fluorescent substance. That is, these light-storing fluorescent substances are usually produced by mixing a mother crystal, and an oxide, a halide or a carbonate of constituent elements as an activator and a coactivator, and calcining the mixture, but it is impossible to avoid that the resulting fluorescent substance is contaminated by a trace amount of impurities. Accordingly, characteristics of the fluorescent substance can vary largely. In other words, to avoid such a problem of contamination with impurities, purification of the oxide, halide or carbonate of constituent elements as a starting material and purification of the resulting fluorescent substance must be conducted carefully. Therefore, the production process is complicated.
Calcination for producing the light-storing fluorescent substance is conducted in the following procedure. That is, starting materials after purification are usually mixed by using a blender, a ball mill or the like, and then the resulting mixture is calcined under a reducing atmosphere. In the case where the mother crystal is an aluminate, the calcination must be conducted at high temperature of not less than about 1300xc2x0 C. However, it is difficult to obtain a crystal having desired high purity, which is also a problem. It is generally desired that the calcination can be conducted at lower temperature and the resulting crystal has high purity, that is, the crystal is composed of a single phase.
After the completion of the calcination, the sintered product is ground by using a crusher, a ball mill or the like to obtain a powdered light-storing fluorescent substance. Since the light emitting characteristics and coating properties are influenced by the particle diameter of the light-storing fluorescent substance, it is necessary to classify for the purpose of removing coarse particles and fine particles after the completion of the grinding. In the solid state reaction method, the scatter in particle diameter is generally large and a tendency to form coarse particles is noticeable.
Furthermore, in the solid state reaction method used conventionally, a flux such as halide of an alkali or an alkali earth metal, boric acid, etc. is often used on calcination for the purpose of promoting crystal growth. Therefore, it is necessary to wash and dry for the purpose of removing the flux and impurities after classification.
In the light-storing fluorescent substance comprising a mother crystal doped with a rare earth metal, which is produced by a conventional solid state reaction method, various necessary conditions described below must be satisfied as described above. That is, a purification step before calcination is required; a flux to be removed in a washing step after calcination must be added in the system; high-temperature calcination is required to obtain a desired crystal form; a classification step is required because the particle diameter of the fluorescent substance obtained after calcination and grinding is not uniform; and a washing step for removing the flux and impurities is required.
An object of the present invention is to solve the above-described problems of the prior art and to provide a light-storing fluorescent pigment, which has high crystal purity and uniform particle diameter and is free from contamination with impurities, without requiring complicated production process and reaction conditions.
Another object of the present invention is to provide a light-storing fluorescent pigment capable of easily controlling the particle diameter and characteristics of the pigment.
Still another object of the present invention is to provide a method of producing such an improved light-storing fluorescent pigment according to demand.
These objects and other objects of the present invention will become apparent from the following detailed description of the present invention.
In one aspect, the present invention provides a light-storing fluorescent pigment comprising a mother crystal and a rare earth element with which said mother crystal is doped, characterized in that said light-storing fluorescent pigment is produced by calcining a gel powder as a starting material obtained from an arbitrary combination of an alkoxide derivative, an organic acid salt, an inorganic acid salt, an oxide and a chloride of metal elements constituting said pigment by using a sol-gel method.
In another aspect, the present invention provides a method of producing a light-storing fluorescent pigment comprising a mother crystal and a rare earth element with which said mother crystal is doped, which comprises the steps of:
producing a gel powder from an arbitrary combination of an alkoxide derivative, an organic acid salt, an inorganic acid salt, an oxide and a chloride of metal elements constituting said pigment by using a sol-gel method; and
calcining the resulting gel under a reducing atmosphere.
In the present invention, as is apparent from the following description, not only a complicated production process required for solid state reaction can be simplified, but also a light-storing fluorescent pigment with a desired crystal form and composition, which has high purity and uniform particle diameter, can be obtained.